This invention relates to modified nematic mixtures having positive dielectric anisotropy.
Nematic liquid-crystal substances are utilized as dielectrics in electrooptic devices in order to convert electric voltage pulses into optically perceptible information. A preferred type of electrooptic device utilizing liquid-crystal substances is the liquid-crystal cells with homogeneous, i.e. parallel or perpendicular orientation, for example a twisted nematic cell arranged between parallel or crossed polarizers such as described in Applied Physics Letters 18: 127-128 (1971), as well as in German Unexamined Laid-Open Application DOS 2,202,555. For such devices, nematic substances are required having positive dielectric anisotropy, i.e. a dielectric constant which is larger when parallel to the longitudinal direction of the molecule swarms than when at right angles to this direction. Those nematic substances are preferred wherein this difference of the two dielectric constants (hereinafter called .DELTA..epsilon.) has a value of at least +2, preferably of at least +3.
Nematic substances having a positive dielectric anisotropy are known. For example, a nematic mixture having this property is described in DOS 2,202,555, consisting of 3-40 parts by weight of a 4-cyanobenzal-4'-alkyl aniline and 97-60 parts by weight of a mixture of (a) 20-80 parts by weight of bis(4'-n-octyloxybenzal)2-chlorophenylenediamine and (b) 20-80 parts by weight of p-methylbenzal-p'-n-butylaniline. According to Applied Physics Letters 18: 127-128 (1971), N-(4'-alkoxybenzylidene)-4-aminobenzonitriles are also utilized as nematic substances having positive dielectric anisotropy.
Although several of these known nematic substances with positive dielectric anisotropy form nematic mesophases over wide and favorably located temperature ranges, i.e. at or below room temperature, they nonetheless have a very limited practical applicability. The azomethines are so sensitive to moisture that partial hydrolysis begins when they are exposed to normal atmospheric humidity. The hydrolysis products reduce the electric resistance of the dielectric and thereby considerably impair the function of the manufactured electrooptic devices.
Other known nematic substances with positive dielectric anisotropy are 4-alkyl-benzoic acid 4'-cyanophenyl esters described in DOS 2,234,522. Although these compounds possess satisfactory chemical stability, they exhibit a narrow nematic mesophase temperature range which is generally so high that electrooptic devices employing these substances must be heated and controlled by a thermostat in the operating condition.
Nematic compositions of azoxybenzenes and phenyl benzoates are known which exhibit excellent chemical resistance and also form nematic mesophases over wide and favorably located temperature ranges, e.g. see DOS 2,014,989 and DOS 2,139,628, respectively.
However, these substances either have a negative or weakly positive (.DELTA..epsilon.&gt;2) dielectric anisotropy and thus cannot be utilized as dielectrics in liquid-crystal cells with homogeneous orientation, i.e. liquid-crystal cells wherein the longitudinal axes of the molecules of the dielectric are orientated parallel to the surface of the electrodes.